Vermicular expanded graphite is a low bulk density, (usually between 0.002 and 0.02 gram per cubic centimeter), particulate, worm-like form of graphite. It is prepared by treating natural flake graphite with an intercalating agent such as fuming nitric acid, fuming sulfuric acid, mixtures of concentrated nitric and sulfuric acid, and the like. The treated graphite is then heated to a high temperature, e.g., above 500.degree. C., to expand the natural flake graphite to the light weight vermicular form. The preparation of the vermicular expanded graphite is well known in the art, and is described in greater detail, for example, in U.S. Pat. Nos. 3,389,964 and 3,323,869, which are incorporated herein by reference.
Vermicular expanded graphite can be mixed with corrosion resistant resins and compressed into various shapes and forms or onto various substrates or supports. The resulting compressed vermicular graphite composite material has many desirable properties including, for example, high electrical and thermal conductivity, high anisotropic ratios, low fluid permeability, resistance to high temperature oxidation, and excellent mechanical properties. Because of these properties, compressed forms of vermicular expanded graphite composite material have found utility as coatings for various metal substrates, for example, as described in U.S. Pat. No. 3,492,197.
Various methods of applying vermicular expanded graphite materials to substrates have been developed. For example, U.S. Pat. No. 3,475,244 describes a process in which compressed vermicular graphite is first treated with a re-expanding agent and heated to produce re-expansion of the graphite surface. The re-expanded graphite is then pressure bonded to the substrate by compression. In another method, vermicular expanded graphite is formed into an integral structure by methods such as those described, for example, in U.S. Pat. No. 3,440,311. The integral structure is then attached to the substrate by use of an adhesive.
One difficulty with these methods is that the resulting vermicular expanded graphite composite structure is especially susceptible to shrinkage and separation at the graphite-substrate interface. Another disadvantage lies in the difficulty of applying a uniform layer of the vermicular expanded graphite composite material to an irregularly-shaped or curved surface. A third disadvantage is that the orientation of the graphite achieved by these methods is often such that there is an undesirable increase in the fluid permeability of the compressed graphite.
Therefore, a need exist for a method of applying a vermicular expanded graphite composite material to the interior surface of a conduit in a manner which results in a uniform coating which is substantially impervious to fluids.